# [SI-LIST] Re: Maximum frequency consideration for high-speed digital analysis of differential signals

• From: <Cortex.Chen@xxxxxxxxxxxx>
• To: <rula.bakleh@xxxxxxxxxx>
• Date: Mon, 9 Jan 2006 12:16:27 +0800

```
Rula,

Maybe you could focus on knee frequency (0.5/Tr, Fknee) instead of BW/f3dB.
It means that the behavior of a circuit at frequencies above Fknee hardly
affects digital performance.

In detail, refer to the section 1.1 of High-Speed Digital Design - A Handbook
of Black Magic (Dr. Howard Johnson's Book@93).

I also have a question for this equation, 0.5/Tr.
It seems to be extracted from the 10% to 90% of Tr.
Who can tell me the knee frequency for 20% to 80% of Tr?

Regards,

Cortex

-----Original Message-----
From: Ed Sayre III [mailto:esayre3@xxxxxxxx]
Sent: Friday, January 06, 2006 11:48 PM
To: Cortex Chen (³¯¥ÃªN)
Cc: tom@xxxxxxxxxxxxx; rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Maximum frequency consideration for high-speed
digital analysis of differential signals

Cortex,

The value of 0.35 in your calculation is base on the pole location of
the 1st order system.  How are you deriving the 0.22 value?.  I am very
curious to understand the thinking behind this.

Regards
-Ed

At 10:25 AM 1/6/2006 +0800, Cortex.Chen@xxxxxxxxxxxx wrote:
>0.35/Tr ---- 10% to 90%
>0.22/Tr ---- 20% to 80%
>
>Regards,
>
>Cortex
>________________________________
>
>From: si-list-bounce@xxxxxxxxxxxxx ¢DN2z Tom Dagostino
>Sent: 2006/1/6 [?P¡¦A?-] ?W?E 10:03
>To: rula.bakleh@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Maximum frequency consideration for high-speed
>digital analysis of differential signals
>
>
>
>I'm not sure how you are computing your bandwidth.  If you have a 25 psec
>risetime (assuming 10-90%) your -3dB bandwidth is 0.35/25psec or 14 GHz.
>
>When analyzing bandwidth requirements for systems like this most people talk
>about passing the 3rd or 5th harmonic of the clock.  In your case that is
>7.5 or 12.5 GHz.  Some analysis of your system requirements is in order.
>
>Tom Dagostino
>Teraspeed(R) Labs
>13610 SW Harness Lane
>Beaverton, OR 97008
>503-430-1065
>tom@xxxxxxxxxxxxx
>www.teraspeed.com
>
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>401-284-1827
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of rula.bakleh@xxxxxxxxxx
>Sent: Thursday, January 05, 2006 2:19 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Maximum frequency consideration for high-speed
>digital analysis of differential signals
>
>
>Hi Everyone,
>
>
>I'm designing a differential signal for the Rocket I/O interface. My signal
>speed is 5 Gbit/sec, operating frequency is 2.5 GHz, and fastest edge rate
>is 25 psec or 40 GHz.
>
>
>
>In regards to performing channel analysis, what is the highest frequency
>content to consider when designing this particular differential signal
>serial link?  If I am measuring or simulating S parameters for various
>portions of this link what is the highest frequency of interest that I need
>to consider as a rule of thumb and why?
>
>
>
>Is it the operating frequency only, multiples/harmonics of the operating
>freq, edge rate frequency (f=1/trise), multiples/harmonics of the edge rate
>freq, or some fraction of the edge rate?  I'm finding conflicting
>information: some people say that it's the full edge rate, others say 0.35
>or 0.5 of the edge rate, and yet others say multiples of the edge frequency
>should be taken into account for analysis and design.  I'm hoping that
>somebody could shed some light on this topic for me.
>
>
>
>Thank You,
>
>Rula Bakleh
>
>
>
>
>
>
>
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